How do I display a message when bootup/shutdown

Clarifications

You ask about procedure and not substance.

I'd have it say, "Check under your desk for food scraps!"

You could change the logon background screen. (Google could help you find)

Or include a line in the logon script that will bring up notepad with the (read-only) file logonserver\\messageoftheday.txt

Or schedule a task for everyone to run at startup

Or program all computers to go into sleep more if untouched for 2 hours

Or distribute green mousepads to everyone

Or do something else

EditAdd:
What about a marquee screensaver: "I'd prefer hibernation"?

here's what it looks like in XP

Start -> Run Secpol.msc

the policy is:

Interactive logon: Message text for users attempting to log on

from the Policy properties dialog;
"Explain this setting" tab

This security setting specifies a text message that is displayed to users when they log on.

This text is often used for legal reasons, for example, to warn users about the ramifications of misusing company information or to warn them that their actions may be audited.

Default: No message.

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You can put any message you want in that setting
that should cover the log on part,
don't know about any settings to display a custom log off / shut down screen
but
I suppose that if they're shutting down the users don't need to see a reminder about shutting down to save power

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this setting is also available in win vista / win7

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this setting can be applied to single workstations or to an entire domain or OU

this MS page covers some basics
http://support.microsoft.com/kb/310430

edit: add / edit info & link